SUN ON THE LINES

Solar Power and the Electric Grid
March 2010 (National Renewable Energy Laboratory)

THE POINT
There are those in the New Energy community who are deeply uncomfortable with Big Energy of any kind, be it Big Oil, Big Coal, Big Wind or Big Sun.

The size of the nation’s energy demand, however, makes “Big” pretty inevitable. The Bigness is driven by a McMansion Dweller-Utility Complex (think Military-Industrial Complex) that has survived the Great Recession and is not likely to turn around until the suburban McMansionites move to yurts. And why should utilities and centralized power providers opt out of the arrangement? They make Big Money controlling energy and fulfilling their end of the bargain by keeping everybody’s lights and televisions and computers on.

Most wind power advocates long ago became accustomed to utility-scale projects and are now producing power at competitive prices. Geothermal, ocean energy and biomass developers have no choice but to think in such proportions if they hope to make their projects pay.

But many solar advocates still think in one-rooftop-at-a-time terms and resent the intrusion of utilities with large scale photovoltaic (PV) and solar power plant (SPP) projects. They contend that solar should be distributed generation and, though it may work in conjunction with the central grid it, it should not be controlled by centralized utilities.

Solar Power and the Electric Grid, from the National Renewable Energy Laboratory (NREL) of the U.S. Department of Energy (DOE), explains the basics of the nation’s power supply and transmission system and describes the most advantageous role solar energy can play in the "bigger" scheme of things.

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The NREL paper explains that solar energy’s variability limits its role as an independent source of power generation. While the variability can be compensated for with costly battery storage and more economy-wide implementation of Energy Efficiency measures, the variability can more cost-effectively be made almost irrelevant by connecting solar systems to the grid in conjunction with the variety of New Energy sources. The distributed generation from individual rooftop solar systems can then, at the same time, protect the grid by smoothing the load peaks. Solar energy is especially well-suited to this because load peaks most often occur at the hottest (and therefore sunniest) part of the day.

Important new work has been published in the last 1-to-2 years demonstrating that current grid capabilities allow for integration of very large amounts of New Energy. The ability to forecast and prepare for weather-induced resource variations and activity-induced load variations is growing all the time. The approaching leap in transmission capability from wider linkage via high voltage wires and more effective linkage via "smart" systems will be enormous. Yet it remains unlikely that any single form of energy generation, even that from the sun’s abundance, will be a final answer.

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THE DETAILS
More and more types of energy are part of the contemporary electricity supply. Most U.S. transmission systems now draw on coal, natural gas, nuclear and – to a more and more significant extent – the New Energies.

While places like Texas and Iowa are getting a growing share of their electricity from wind power, many states are preparing to put more solar energy-generated electricity into their power mix.

The grid interconnects an increasingly varied energy supply (generation) with the increasingly demanding population of electricity users (load). Less than a century ago, most citizens did NOT have electricity; now living off the grid in the U.S. is an oddity.

Solar started as offgrid panels but has evolved into grid-integrated energy. (click to enlarge)

Large quantities of electricity cannot be cost-effectively stored so grid operators are constantly working to match generation to load. Over the course of a day and through the seasons of the year, they ramp down generation as demand falls off, ramp it up as demand begins rising and scramble for more as demand peaks.

Though the grid started out as balkanized loops in service to localized areas and carried largely coal-based generation, it has matured to serve larger regions and carry a more varied generation. This allows grid operators to manage a fleet of generation sources and balance more varied loads to get optimal performance.

Using the grid to manage load and generation makes higher reliability possible. It expands the reach of grid operators and allows them to bring more types of generation into their mix. It leads to more power at a lower cost for electricity consumers.

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Each of the New Energies fits into the load curve in a different way. Geothermal, biomass and ocean energy can be available upon need but are not at present likely to be available in large quantities.

Wind is available at an increasingly large scale – it was the second biggest source of new installed capacity in 2009 and has been for the last 4 years – but it is variable. It’s best use is in conjunction with other energy sources and over a wide-ranging enough transmission system so that wind not blowing in one place can be replaced by wind blowing in another.

Solar is especially valuable to grid operators in this context. The other New Energies are less compromised in their availabilities but take time to ramp up and down. This makes them good sources to meet predictable “intermediate loads.”

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Solar energy, on the other hand, is - when the sun is shining - promptly dispatchable. It can be integrated into or out of the grid’s generation supply quickly and easily. This makes it a superb peak load smoothing source, especially because the daily and seasonal peak loads often occur when the electricity demand for air conditioners suddenly rises because the sun is hottest and brightest.

New skills at grid integration and the forecasting of weather and demand make the use of variable New Energies more practical than it has ever been before. Generation fleet diversity adds up to reliability through the capability of drawing on a variety of opportunities.

(1) “Must-take” New Energy generation can be used to reduce reliance on the Old Energies whenever they are available. The best example is wind power. Any output available can readily be integrated at competitive costs into the grid’s supply until it reaches proportions of the generation mix above 30%-to-40%, a level of production it is not likely to achieve until U.S. transmission technology is ready to manage it.

(2) Solar photovoltaic (PV) generates electricity from the light of the sun in centralized plants or distributed on rooftops. PV serves grid operators’ "peak load" needs without requiring increased land or new transmission. As PV becomes more price competitive and is installed in larger quantities, it will also act as “must-take” generation.

(3) Solar power plants (SPPs) that use concentrating solar power (CSP) gather the heat of the sun and use it to boil water and create steam to generate electricity. They are being developed with the capacity to store the heat in the form of compressed hot water or molten salts. This storage capability moves solar energy "peak load" service into the role of less variable, more dispatchable intermediate “must-take” load generation and will likely eventually allow it (along with stored wind energy-generated electricity) to be a "baseload" resource like the Old Energies, geothermal energy and biogas produced from biomass.

NREL studies show it is possible to effectively integrate large portions of New Energy into the grid supply. (click top enlarge)

The various New Energies, used in appropriate combinations at appropriate locations – and in conjunction with aggressively deployed Energy Efficiency – offer the potential to significantly reduce dependence on fossil fuels and the ills that accompany them without compromising power supply reliability. But right now the New Energies do not have all the characteristics needed to fully meet grid supply needs. Without electricity storage, distributed PV generation is too variable and with electricity storage it is still prohibitively costly.

NREL’s assessment of available rooftop space shows PV could meet 10%-to-25% of U.S. electricity needs. Large-scale, ground-mounted systems would be more cost-effective. But even factoring the cost of transmission, centralized PV and SPPs are the least costly use of solar energy because they allow economies-of-scale in construction and operation and can be located in areas of highest insolation.

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PV without cost effective electricity storage does not provide a stable grid supply but is a valuable adjunct that facilitates grid operation. It provides no electricity when there is no sun and an erratic supply in cloudy weather. (SPPs allow for storage and respond more evenly to cloud covers.) Like other variable sources, PV puts an extra burden on the grid operator to provide backup generation.

But these things will not permanently remain obstacles to solar and the other New Energies. Economies of scale obtained from production for grid-tied rooftop systems will bring costs for systems and storage down. The advent of the Smart Grid on a wide scale will allow for more efficient New Energy integration. More effective use of better weather and load forecasting will eliminate many of the "biggest" obstacles to greater use of PV solar specifically and the New Energies generally. Eventually, sun, wind and water will serve; this is a transitional time.

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QUOTES
- From the NREL paper on solar energy and the transmission system: “The electric grid is a complex network that is an integral part of our society. Running the grid in the presence of increasing fuel costs and growing environmental concerns will require new technologies and ways to use them. While renewable power technologies will be an essential part of our energy future, no one technology can provide all of the energy and services we need. Careful integration of distributed generation and careful deployment of utility-scale generation will be needed to provide the mix of power and reliability that we require for a healthy electric supply as renewables contribute an increasingly larger share of our energy needs.”

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- From the NREL paper on solar energy and the transmission system: “The level of demand for electricity in any one area is so variable that it is more efficient to combine demand from many sites into an overall regional load. This regional electric load is then met by the output of a fleet of generators that can be controlled and managed for optimal performance. In part, the grid was developed to allow generators to provide backup to each other and share load…The grid also allows generators to be located closer to resources (e.g., fuel supply, water, available land) and ship electricity over the transmission and distribution network to different load centers. Utility-scale solar and wind power plants are conceptually similar to conventional generators—they generate electricity where the necessary resources are located, typically in remote areas where the fuel (sunlight or wind) is most abundant. These attributes—consolidating variable individual loads into more predictable regional loads, siting plants near their resource base, and extensive transmission lines—help the grid provide electric power with good reliability and low cost.”